DE102012022343B4 - Method for monitoring a layer growth and device for coating - Google Patents

Abstract

A method of monitoring a layer growth during coating of a surface of a substrate (48), the method comprising the steps of:
a) Performing interference measurements
- Wherein two partial beams (36, 38) of electromagnetic radiation of a radiation source (32) are brought into interference with each other, of which one (38) has been reflected on the surface and
- electromagnetic radiation with a large number of different frequencies is used,
b) determining a spectral phase Φ from the interference measurements and
c) calculating from the spectral phase Φ at least one control variable that allows a statement about the degree and / or the quality of the layer growth, wherein the substrate (48) moves.

Description

The invention relates to a method for monitoring a layer growth during the coating of a surface of a substrate. The invention also relates to a device for coating a surface of a substrate.

Coatings of surfaces of substrates with particular layer materials are used today in many different applications in various fields of technology in order to change the properties, for example the optical properties of the surface to the desired extent. In order to achieve this, in particular the applied layers must be applied very homogeneously and as far as possible without defects and defects, and moreover must have as precisely as possible a predetermined thickness.

In particular, in the production of dispersive mirrors, which are used for example when working with short laser pulses, a large number, for example forty to fifty, pairs of layers of different materials are applied to one another. Dispersive mirrors are often designed for the respective application, so that they have exactly the properties required for the respective intended use. For this purpose, the required layer thicknesses of the different materials are calculated in advance and must be kept as accurately as possible during production. Here, as in the EP 1 536 529 B1 is calculated, the layer thicknesses down to the sub-nanometer range calculated precisely to achieve the desired properties of the dispersive mirror.

In particular, in the production of dispersive mirrors, but also in many other applications of thin coatings on the surfaces of substrates, it is therefore important to comply with the desired layer thicknesses as accurately as possible in order to achieve the desired properties of the surface.

To achieve this, a number of methods are known from the prior art.

For example, it is possible to trace the mass deposited and deposited on the surface of the substrate during the coating by means of a quartz oscillator. Due to the very low layer thicknesses required in particular for dispersive mirrors and the associated very low deposited mass, this method is limited in its accuracy. Another possibility is to consider the change in the spectral transmission curve of the applied layer during the coating process. However, this is only possible to a limited extent due to the thin layers to be applied. From the essays "In-situ broadband monitoring of heterogeneous optical coatings", in Thin Solid Films, Vol. 502, pp. 153-157 by Wilbrandt et al. and "Synthesis and manufacturing the mirrors for ultrafast optics" in Advances in Optical Thin Films II, Proc. of SPIE Vol. 5963, 59631P, (2005) by Pervak et al. as well as from the essay "Substantial progress in optical monitoring by intermittent measurement technique" in Advances in Optical Thin Films II, Proc. of SPIE Vol. 5963, 59630D, (2005) by Zoeller et al. For example, methods for the production of dispersive mirrors are known in which the layer thickness of the applied layers is determined by an in-situ measurement of the transmission. In the article Spectral interferometry: Comparision of different signal processing algorithms ", in APPLIED OPTICS, Vol. 50, No. 9, pages C239 - C245 by Luo et al. Instead, a combination of a determination of the deposited mass using a quartz crystal and a measurement of the duration of the evaporation of the substrate is used.

In the article "Spectral-phase interference method for detecting biochemical reactions on a surface", Quantum Electronics 30 (12), pp. 1099-1104 (2000) The aim is to detect biological molecules that adhere to a previously prepared glass plate.

In the essays "Properties of chirped mirrors manufactured by plasma ion assisted electron beam evaporation", in Advances in Optical Thin Films II, Proc. of SPIE, Vol. 5963, 59631 N, (2005) by Bischoff et al. however, it is described that after the completion of the layer to be applied, the group delay or a group velocity dispersion are measured. How to perform such a measurement is, for example, in the article "Dispersion measurements with white-light interferometry" in J. Opt. Soc. At the. B / Vol. 13, no. 6, pages 1120 - 1129 described by Diddams and Diels. In the article "Advanced production of thin film laser components" in Advances in Optical Thin Film II, Proc. of SPIE Vol. 5963, 596319, (2005) by Scherer et al. It is described that the structure of dispersive mirrors in situ can not be satisfactorily monitored.

In the production of complicated layer structures whose layer thicknesses must be kept as precisely as possible for each of the layers, it is not sufficient to determine the exact layer thicknesses only after the coating, since this merely represents a control of the coating process, but not used to control the coating can. So it is not possible, the layer thicknesses of already applied layers To change later to adapt to the desired thickness course. Methods which determine the layer thicknesses on the already finished layer structure are therefore completely unsuitable for this purpose.

In recent years, so-called optical coherence tomography (OCT) has become established as a method by which information from different depths of a material can be extracted. This is of great interest, for example, in medical technology, since information about materials and structures can be extracted at different depths, for example under the human skin. In optical coherence tomography, an interference signal is recorded in which two partial beams of electromagnetic radiation are brought into interference in an interferometer. One of the two partial beams is thereby reflected on the object to be examined. In this case, electromagnetic radiation of a multiplicity of different wavelengths, for example coming from a white light source, is used. In the thus determined interferogram is the depth information, which can be extracted by suitable processing. Insights into deeper layers of a layer system or an organic system are thus possible without destruction taking place. By means of the OCT technology, it is possible to penetrate several hundred microns deep into a material and to obtain information about existing layer structures or similar three-dimensional structures. An apparatus for performing an optical coherence tomography, for example, in US 2011/0216325 A1 described. However, to monitor the coating process, in particular for dispersive mirrors, this method has only limited suitability. The layer structures to be detected and resolved in this case are usually a few 10 nm thick and must be exactly resolved to fractions of nm. This is only possible to a limited extent by optical coherence tomography in the conventional sense.

The present invention is therefore based on the object of proposing a method for monitoring a layer growth during the coating of a surface of a substrate, which allows an accurate, fast and reliable determination of the quality and / or the degree of coating, so that the sizes thus determined Control of the coating process can be used. In addition, an apparatus for carrying out such a method is to be proposed.

1. a) Durchführen von Interferenzmessungen,
   • - wobei zwei Teilstrahlen elektromagnetischer Strahlung einer Strahlungsquelle miteinander zur Interferenz gebracht werden, von denen einer an der Oberfläche reflektiert wurde und
   • - elektromagnetische Strahlung mit einer Vielzahl unterschiedlicher Frequenzen verwendet wird,
2. b) Ermitteln einer spektralen Phase aus den Interferenzmessungen und
3. c) Berechnen von wenigstens einer Kontrollgröße, die eine Aussage über den Grad und/oder die Qualität des Schichtwachstums aus der spektralen Phase erlaubt.

The invention achieves the stated object by a generic method which has the following steps:

1. a) performing interference measurements,
   • - Wherein two partial beams of electromagnetic radiation of a radiation source are brought into interference with each other, one of which has been reflected on the surface and
   • - electromagnetic radiation with a large number of different frequencies is used,
2. b) determining a spectral phase from the interference measurements and
3. c) calculating at least one control variable that allows a statement about the degree and / or the quality of the layer growth from the spectral phase.

This process is therefore performed directly during the coating process, that is, while material is deposited or deposited on the substrate. It is therefore an in situ measurement which is therefore well suited to monitor the progress of layer growth and, if necessary, to adjust parameters of layer growth. The method has the great advantage that control quantities can be calculated from the spectral phase, which allow a much more detailed and accurate statement about the degree of layer growth, as would be possible, for example, by the pure amplitude evaluation of optical coherence tomography.

By performing interference measurements is meant any determination of a distribution of the power density of the electromagnetic radiation at the output of an interferometer. The distribution can be recorded spatially, temporally and / or spectrally. In particular, the taking of interference measurements is understood as the recording of an interference spectrum.

It has proven to be advantageous if the control variable is a group delay which is calculated by deriving the spectral phase once after the optical frequency. It is also advantageous if the control variable additionally or alternatively to a group velocity dispersion, which is calculated by deriving the spectral phase twice after the optical frequency.

Particularly in the production of dispersive mirrors, these two parameters, ie the group delay and the group velocity dispersion, are the decisive properties of the layer structure. The layer structure of a dispersive mirror has been designed and calculated to achieve a certain group velocity dispersion or group delay. Because these values are in situ from the Spectral phase of the interference spectrum can be calculated, these values can be compared directly with these crucial variables of a dispersive mirror. In addition, they change even with only partial and incomplete coating, so that a partial overlap of the substrate with the applied layer can be detected.

In method step a), for example, an interference spectrum is recorded, wherein electromagnetic radiation of different frequency is used. It is actually the recording of many partial interference spectra, each of which uses electromagnetic radiation of a certain frequency. By combining these partial interference spectra with electromagnetic radiation of different frequencies, it is possible to determine the complete interference spectrum from which the spectral phase can subsequently be determined.

It has proven to be advantageous if the radiation source is a broadband radiation source which emits electromagnetic radiation with a multiplicity of different frequencies simultaneously. This may be, for example, a superluminescent diode, a mode-locked laser, a halogen lamp or a glass fiber-based white light source. Consequently, the electromagnetic radiation emitted by such a broadband radiation source does not have a single but a multiplicity of different frequencies. Therefore, it is possible to simultaneously record a plurality of the partial interference spectra. Therefore, the apparatus design for a device for carrying out such a method is particularly simple and can be easily implemented even with commercially available coating chambers.

Alternatively, a tunable radiation source that emits only narrow band electromagnetic radiation may be used. However, this can be tuned over a certain frequency range, so that the different partial interference spectra can be recorded one after the other. However, the expenditure on equipment for this solution is greater, since on the one hand a tunable radiation source and on the other hand a fast photodetector must be used, which detects the different partial interference spectra, and forwards them for further processing. However, the advantage of such a design lies in the greater speed and the better signal-to-noise ratio.

In addition, in this embodiment eliminates the need to use a spectrometer, so that the equipment cost is further reduced.

• b1) Fourier-Transformieren eines aus den Intereferenzmessungen erhaltenen Interferenzspektrums von einem Frequenz-Raum in einen Zeit-Raum, um ein transformiertes Interferenzspektrum zu erhalten,
• b2) Filtern des transformierten Interferenzspektrums durch Multiplizieren mit einer Filterfunktion,
• b3) Rücktransformieren des Interferenzspektrums aus dem Zeit-Raum in den Frequenz-Raum,
• b4) Bestimmen der spektralen Phase.

The determination of the spectral phase advantageously has the following steps:

• b1) Fourier transforming an interference spectrum obtained from the interference measurements from a frequency space to a time space to obtain a transformed interference spectrum;
• b2) filtering the transformed interference spectrum by multiplying by a filter function,
• b3) retransforming the interference spectrum from the time space to the frequency space,
• b4) Determining the spectral phase.

After the back-transformation of the interference spectrum into the frequency space by a second Fourier transformation, the spectral field amplitudes, which are determined in this way, are complex-valued. From this it is easy to determine the spectral phase, from which all other quantities can be calculated. By multiplying by a filter function, it is possible to filter out especially the higher order of the interference and the higher harmonics and thus to clean up the spectrum.

After multiplication, the filtered transformed interference spectrum can be shifted to a time zero. For this purpose, for example, a new time scale is introduced, which is shifted by a specific time interval compared to the actual time scale. In this way, the result of the later transformation to be carried out from the period is influenced in the frequency domain.

Advantageously, the filter function is adapted to the applied coating and / or the substrate. Thus, it may be advantageous for different substrates and / or different coatings to use different forms of filter functions. In addition to the simple rectangular filter in which simply a part of the transformed interference spectrum is filtered out, more complex shaped filter functions, such as supergauss functions, are conceivable in which different portions of the transformed interference spectrum are used differently weighted. The optimum shape of the filter function used depends strongly on the type and the layer thickness of the applied layer material and the underlying substrate.

According to the invention, the method is carried out while the substrate is moving. The substrate is in the coating process in the coating chamber, in which it is on a Substrate holder is attached and rotates advantageously to ensure a particularly uniform coating. In order to actually be able to perform an in-situ measurement here and not have to interrupt the actual coating process by measuring breaks, the process can be carried out while the substrate is still moved in the coating chamber. In order to record the interference spectrum, only a short pulse of the electromagnetic radiation has to be split into two partial beams, one of which is reflected at the surface to be measured. The pulse duration is so low that the movement of the substrate and the coating in this short time that the surface is illuminated by the electromagnetic radiation is not perceived. The measurement times are in the sub-millisecond range, so that the movement of the substrate, which lies in a plane perpendicular to Einstrahlebene the electromagnetic beam part, does not matter. In addition, the measurement can be performed synchronized with the movement of the substrate, which is preferably a rotation.

Also, an optionally present wobbling movement, which is carried out by the substrate due to, for example, imperfect attachment to the substrate holder, merely results in that not all of the electromagnetic radiation incident on the surface to be measured can be reflected back and collected by the detector. Part of the electromagnetic radiation is lost, which reduces the amplitude of the spectrum. However, since in the present method, unlike optical coherence tomography, the amplitude of the spectrum is only of secondary importance, this loss of electromagnetic radiation is not serious.

A device according to the invention for coating a surface of a substrate comprises a coating chamber, an interferometer, arranged to cause two partial beams of electromagnetic radiation to interfere with each other, one of which has been reflected at the surface, a radiation source for electromagnetic radiation with different frequencies, means for moving the Substrates and an electrical controller configured to perform a method described herein. Advantageously, the electrical control is also set up to control operating parameters as a function of the calculated control variable. This is particularly advantageous if the calculated control variable is the group delay and / or the group velocity dispersion. As already stated, a dispersive mirror for a single application is designed to have a desired group velocity dispersion. The fact that this size can be determined by the method described here with the device described here quickly, easily and reliably directly from the recorded interference spectrum, a particularly direct control of the coating process can be achieved. Consequently, the control variables calculated from the spectral phases can be compared directly with the theoretically calculated desired values and thus the coating method can be easily controlled. One possible operating parameter which can be controlled in this way is, for example, a coating rate with which the surface of the substrate is coated.

• 1 - eine exemplarische Darstellung eines Interferenzspektrums,
• 2 - eine beispielhafte Darstellung eines transformierten Interferenzspektrums mit anzuwendender Filterfunktion,
• 3 - die exemplarische Darstellung spektraler Phasen,
• 4 - aus den spektralen Phasen aus 3 errechnete Gruppenverzögerungen
• 5 - aus den Gruppenverzögerungen aus 4 errechnete Gruppengeschwindigkeitsdispersionen,
• 6 - den schematischen Aufbau einer Vorrichtung gemäß einem ersten Ausführungsbeispiel der vorliegenden Erfindung und
• 7 - den schematischen Aufbau einer Vorrichtung gemäß einem zweiten Ausführungsbeispiel der vorliegenden Erfindung.

With the aid of a drawing, an embodiment of the present invention will be explained in more detail below. It shows

• 1 an exemplary representation of an interference spectrum,
• 2 an exemplary representation of a transformed interference spectrum with filter function to be used,
• 3 the exemplary representation of spectral phases,
• 4 - from the spectral phases 3 calculated group delays
• 5 - from the group delays 4 calculated group velocity dispersions,
• 6 - The schematic structure of a device according to a first embodiment of the present invention and
• 7 - The schematic structure of a device according to a second embodiment of the present invention.

1 shows an interference spectrum 2 as received in a first method step in a method according to an embodiment of the present invention. A spectral power density is plotted I over a wavelength λ , The wavelength can be determined by the dispersion relation λ easily converted into a frequency. This in 1 shown interference spectrum 2 has been recorded in a single location and shows that it depends on the wavelength λ the electromagnetic radiation used is due to the different interference conditions to different intensities at the detection element.

After the interference spectrum 2 from a function that depends on the wavelength λ is converted into a function which depends on the frequency of the electromagnetic radiation, this function can be converted into the time-space by a first Fourier transformation. This is in 2 shown. One recognizes a transformed interference spectrum 4 , where again on a logarithmic scale the intensity I over time t is applied. It can be seen that it is next to a peak at t = 0 at the very left end of the in 2 shown diagram further prominent structures 6 of which one through a filter function 8th is picked out. Advantageously, this is the first structure 6 , which is the sharpest and strongest trained. By filtering out this structure 6 through the filter function 8th higher harmonics and higher order interferences are eliminated. In the 2 shown filter function 8th is strongly approximated to a rectangular filter. Note again the logarithmic representation along the Y-axis. However, here are other forms of filtering functions involving different parts of the transformed interference spectrum 4 possible with different weights.

After the transformed interference spectrum 4 with the filter function 8th multiplied, and thus only the first structure 6 in the transformed interference spectrum 4 remains after the shift to time zero, a re-Fourier transform is preferably carried out to convert the interference spectrum back into the frequency space. This results in complex spectral field amplitudes, from which now simply the spectral phase Φ can be determined. This is in 3 represented and as a function of the wavelength λ applied. In the middle, the Φ = 0 line is indicated by a dashed line 10 demonstrated. Different interference spectra provide different spectral phases. By comparing the spectral phase with a theoretically determined desired phase curve, it can be determined how far the coating applied to a substrate coincides with the desired coating. While one at the top of the 3 illustrated first phase curve 12 from a previously theoretically determined desired phase curve shown in the lower area 14 is still relatively far away, is a second phase course 16 already in relatively good agreement with the target phase course 14 , The two phase curves 12 . 14 In this case, for example, interference spectra which were recorded at different times of the coating correspond.

4 shows a diagram in which a group delay DG over the wavelength λ the electromagnetic radiation used is applied. You get the group delay DG by once deriving the phase curve according to the optical frequency. 4 shows a first group delay 18 that comes from the first phase 12 was calculated. In addition, there is a target group delay 20 and a second group delay 22 shown from the target phase 14 or the second phase curve 16 was calculated. Again, one recognizes that the coating is the first group delay 18 has led, from the target coating, the target group delay 20 is still very far away. The second coating leading to the second group delay 22 leads, is already much closer to the target coating, but also has significant deviations from the target group delay, especially at high wavelengths 20 on. A comparison with 3 shows that the deviations at high wavelengths λ in the group delay DG become clearer.

This becomes even more apparent in 5 in which a group velocity dispersion GDD as a function of wavelength λ is applied. The group velocity dispersion is obtained GDD by deriving the group delay once DG according to the optical frequency. The group velocity dispersion GDD is therefore best suited for controlling the coating process.

5 shows a first group velocity dispersion 24 , a target group velocity dispersion 26 and a second group velocity dispersion 28 , These were from the first group delay 18 , the target group delay 20 and the second group delay 22 calculated and show how the 3 and 4 in that the first coating is the first group velocity dispersion 24 leads, is still clearly removed from a target coating.

The great advantage of such a method is that, on the one hand, a very accurate and rapid possibility has been created to control the coating process, since the method can be carried out during the deposition of material on the substrate surface. It can thus be seen during the coating, how far the layer growth has already progressed or whether certain parameters of the coating chamber must be changed. On the other hand, in particular, the complicated layer structures of dispersive mirrors are calculated and designed in such a way that they have a specific phase progression, a precalculated group delay or group velocity dispersion. Due to the fact that these quantities can be calculated easily and quickly from the recorded interference spectrum, there is a direct possibility of controlling and controlling the coating process.

6 shows the schematic representation of a device 30 , according to an embodiment of the present invention. That from a broadband or broadband tunable light source 32 emitted light is irradiated in an interferometer and at a beam splitter 34 in a first partial beam 36 and a second sub-beam 38 split. The first partial beam passes through a refractive index corrector 40 and then hits a reference mirror 42 from which it is again towards the beam splitter 34 is reflected. The second partial beam 38 enters a vacuum coating chamber 44 one, where he has a vacuum window 46 passes. The refractive index of the vacuum window 46 ensures a slightly different optical path length, which is due to the refractive index corrector 40 that in the first part of the beam 36 is arranged is compensated.

Subsequently, the second partial beam hits 38 on a substrate 48 that in a substrate holder 50 is arranged, extending along the direction of the arrow 52 rotates.

The second partial beam 38 gets from the substrate 48 or reflected on him partial coating reflects, passes through the vacuum window again 46 and hits the beam splitter again 34 , Both partial beams 36 . 38 then become a fast spectrometer 54 fed that the interference spectrum 2 receives. The spectrometer 54 is with an electrical control 56 configured to perform a method according to an embodiment of the present invention.

7 shows the device 30 according to another embodiment of the present invention. From the light source 32 the light is transmitted through an imaging optics 60 in an optical fiber 62 coupled and a fiber coupler 64 fed, which acts as a beam splitter. The first partial beam 36 is via another optical fiber 62 and another imaging optics 60 on the reference mirror 42 guided and reflected there. The second partial beam 38 is also via optical fibers 62 into the vacuum coating chamber 44 led and meets there for another imaging optics 60 on the substrate 48 that in the substrate holder 50 is arranged and moving along the arrow 52 rotates. The two partial beams 36 . 38 go through the optical fibers 62 again and become the fiber coupler again 64 fed and then via another optical fiber 62 and another imaging optics 60 the spectrometer 54 supplied, as in 6 illustrated embodiment with the electrical control 56 configured to execute a method according to an embodiment of the present invention. The optical fibers are optionally via polarization actuators 66 guided.

LIST OF REFERENCE NUMBERS

I

intensity

λ

wavelength

t

Time

Φ

Spectral phase

DG

group delay

GDD

Group velocity dispersion

2

interference spectrum

4

Fourier-transformed interference spectrum

6

structure

8th

filter function

10

Dashed line

12

First phase progression

14

Should phase response

16

Second phase course

18

First group delay

20

Target group delay

22

Second group delay

24

First group velocity dispersion

26

Target group velocity dispersion

28

Second group velocity-Keitsdispersion

30

contraption

32

light source

34

beamsplitter

36

First partial beam

38

Second partial beam

40

Refractive index corrector

42

reference mirror

44

Vacuum coating chamber

46

vacuum window

48

substratum

50

substrate holder

52

arrow

54

spectrometer

56

Electric control

58

rotary encoder

60

imaging optics

62

optical fiber

64

fiber coupler

66

Polarization control element

Claims (9)

A method of monitoring a layer growth during coating of a surface of a substrate (48), the method comprising the steps of: a) Performing interference measurements - Wherein two partial beams (36, 38) of electromagnetic radiation of a radiation source (32) are brought into interference with each other, of which one (38) has been reflected on the surface and - electromagnetic radiation with a large number of different frequencies is used, b) determining a spectral phase Φ from the interference measurements and c) calculating from the spectral phase Φ at least one control variable that allows a statement about the degree and / or the quality of the layer growth, wherein the substrate (48) moves. Method according to Claim 1 , characterized in that the control quantity is a group delay (GD) which is calculated by once deriving the spectral phase Φ according to the optical frequency. Method according to Claim 1 or 2 , characterized in that the control quantity is a group velocity dispersion (GDD) which is calculated by deriving the spectral phase Φ twice according to the optical frequency. Method according to Claim 1 . 2 or 3 , characterized in that the radiation source (32) is a broadband radiation source which simultaneously emits electromagnetic radiation at the plurality of different frequencies. Method according to Claim 1 . 2 or 3 , characterized in that the radiation source (32) is a tunable radiation source. Method according to one of the preceding claims, characterized in that the determination of the spectral phase Φ comprises the following steps: b1) Fourier-transforming an interference spectrum (2) obtained from the interference measurements from a frequency space to a time space to form a transformed interference spectrum (4) to obtain b2) filtering the transformed interference spectrum (4) by multiplying by a filter function (8), b3) inverse transforming the transformed interference spectrum (4) from the time space to the frequency space, b4) determining the spectral phase Φ , Method according to Claim 6 , characterized in that the filter function (8) is adapted to the applied coating and / or the substrate (48). Apparatus (30) for coating a surface of a substrate (48), the apparatus (30) comprising: a coating chamber (44) an interferometer arranged to cause two partial beams (36, 38) of electromagnetic radiation to interfere with each other, one of which (38) has been reflected at the surface, a radiation source (32) for electromagnetic radiation of different frequencies, Means for moving the substrate and - An electrical control (56) which is adapted for carrying out a method according to any one of the preceding claims. Device (30) according to Claim 8 , characterized in that the electrical control (56) is arranged to control operating parameters as a function of the calculated control variable.

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